1. Field of the Invention
The present invention relates to an image heating apparatus which is suited for use as a heating fixing apparatus of an image forming apparatus using a recording technique such as an electrophotographic recording system, an electrostatic recording system, or the like, and in particular, to an image heating apparatus forming a nip part which nips a flexible rotating body therebetween and nip-conveys a recording material by a sliding member and a pressure roller.
2. Description of Related Art
For convenience, an example of an image heating apparatus (fixing apparatus), which is provided at an image forming apparatus such as a copier, a printer, or the like and which heat-fixes a toner image on a recording material, will be described.
In the image forming apparatus, a heat roller system apparatus has been broadly used as a fixing apparatus which heat-fixes, as a permanently fixed image and onto a recording material surface, an unfixed image (toner image) of object image information formed and borne by a transfer system or a direct system on a recording material (a transfer material sheet, an electro fax sheet, an electrostatic recording paper, an OHP sheet, a printing paper, a format paper, or the like) at a proper image forming process means section such as an electrophotographic process, an electrostatic recording process, a magnetic recording process, or the like.
Recently, from the standpoints of quick starting and saving energy, a film heating system apparatus has been put into practical use. Further, an electromagnetic induction heating system apparatus, in which a film itself generates heat, has been proposed.
In recent years, high speed has been strongly required of the fixing apparatus, and at the same time, making the image forming apparatus compact has been required. Further, the demand for color image forming apparatuses has increased. When a fixing roller and a pressure roller having small diameters are used to make an apparatus compact and the apparatus is a color image forming apparatus and the speed of conveying a recording material is fast, in order to permanently fix an unfixed toner image on a recording material surface, there is the need to apply a sufficient amount of heat and pressure to the recording material.
In a film heating system fixing apparatus having a metal layer, as compared with a resin film type fixing apparatus, there are features that the strength is high and the thermal conductivity is high, and it is easy to correspond to high-speed processing.
In Japanese Patent Application Laid-Open No. 7-114276, there is disclosed a heating apparatus which generates eddy current at a film itself or at a conductive member set close to the film, and heats by Joule heat.
This electromagnetic induction heating system can achieve an increase in the efficiency of consumed energy and also can correspond to high-speed processing, because the film itself is heated.
In a film heating system heating apparatus or an electromagnetic induction heating system heating apparatus using a film, as the method of driving a cylindrical or endless film shaped film serving as a rotating body, there are a method in which a film, which is pressed by a pressure roller and a film guide member guiding the film inner peripheral surface, is slave-rotated by rotation-driving the pressure roller (a pressure roller driving system), and conversely, a method in which the pressure roller is slave-rotated by driving of an endless film shaped film stretched between a drive roller and a tension roller.
However, in a heating fixing apparatus which uses a film having a metal layer as a rotating body, when a paper which is a recording material passes through the fixing apparatus, there are cases in which wrinkles arise at the paper at the fixing nip part. It is easy for wrinkles to arise in particular at thin papers.
Conventionally, in a heat roller system heating fixing apparatus which is generally used, by making the outside diameter of the fixing roller have an inverse crown shape, a force stretching a paper forward both sides is generated by making the paper conveying speed at the fixing nip part fast at the both edge parts and slow at the central part. A technique is thereby used which prevents the occurrence of paper wrinkles.
On the other hand, in the heating fixing apparatus which uses the film having a metal layer as a rotating body, it is difficult to make the fixing film corresponding to the heat roller to have an inverse crown shape. Further, the occurrence of paper wrinkles can be prevented by making an inverse crown shape at the sliding part of the fixing film and a fixing film guide member holding the fixing film. However, the shape of the fixing nip is an extreme central-concave shape, and troubles arise such as glossiness at the central portion of the image decreases or the like.
Further, when the force stretching the paper toward both sides in order to prevent paper wrinkles is too strong, there are cases in which the trailing edge of the paper springs up, and image defects such as rubbing or the like are caused.
The prevent invention has been achieved in consideration of the above-described problems, and an object of the present invention is to provide an image heating apparatus which can reduce wrinkles of a recording material and springing up of the conveying direction trailing edge of the recording material.
Another object of the present invention is to provide an image heating apparatus which can carry out uniform heating of an image while suppressing wrinkles of a recording material and springing up of the conveying direction trailing edge of the recording material.
Yet another object of the present invention is to provide an image heating apparatus comprising:
a flexible rotating body (rotating member);
a sliding member which is disposed at an interior of the rotating body and slides with the rotating body; and
a pressure roller which, together with the sliding member, forms a nip part via the rotating body,
wherein a sliding surface of the nip part of the sliding member is crown shaped along a longitudinal direction of the sliding member, and the pressure roller is an inverse crown shape along the longitudinal direction, and between a crown amount cr per unit length of the crown shaped portion of the sliding member and an inverse crown amount crxe2x80x2 per unit length of the pressure roller, there is the relationship:
1.00xc3x9710xe2x88x923xe2x89xa6crxe2x88x924crxe2x80x3xe2x89xa64.0xc3x9710xe2x88x923. 